Adding guinea pig serum, its natural or heated fractions to the antigens, their effects on precipitin reaction have been studied. As the antigen, polysacchalides from yeast have been used, and the antiserum has been obtained from rabbit immunized by it. The results were as follows: 1) Uhlenhuth's precipitin reaction by the antigen diluted with heated serum or heated albumin fraction shows higher titer than routine method, while the reaction by the antigen diluted with natural (unheated) serum or the unheated albmin fraction does not show any effect to increase the titer. The globulin fraction shows no effect whether it is heated or not. 2) In Ogata's reaction (antigen-antibody diluting method), both guinea pig serum and its albumin fraction have the increasing effect. The globulin fraction has the effect if it is heated, but not if it is used unheated. 3) The fractions by electrophoresis, albumin, α(1) -, α(2) -, β -, γ - globulin have the effect of following order. albumin > α(2) - ≧ α(1) - > β - > γ - globulin 4) Guinea pig serum and its albumin fraction shows remarkable turbidity by heating, but its globulin fraction does not show any difference among turbidities caused by heating (from 56℃ to 62℃).